parametr pass to <% control %>

Hi!
my problem is understand what kind of object pass to <% control %>.
for example I have this: $data = array('Nome' => $data['nome']);
So I can use :
<% if Nome %> and <% control Nome %>
Why?
the same is valid if I use a name of function? can you explain me?I read http://doc.silverstripe.com/doku.php?id=templates&s=renderwith but is not clear